Ethylated and methylated sterols predominate in plant and fungal cell membranes, respectively, whereas animal cell membranes contain non-alkylated sterols. While the most important role of sterols is to modulate membrane fluidity, recent investigations in the yeast, Saccharomyces cerevisiae, indicating that only microamounts of highly specific sterols are essential for growth suggests an additional but as yet undetermined role of sterols. In this regard, the absolute requirement for methylated sterols in yeast has been debated in the literature. The aim of this proposal is to determine whether an absolute requirement for methylated sterols exists by first cloning the yeast sterol transmethylase (SMT) gene and then using gene disruption techniques to insert a DNA fragment into the chromosomal SMT gene rendering it inactive. Gene disruption of the chromosomal SMT gene will be confirmed by genetic, biochemical and recombinant DNA analyses. If it is determined that methylated sterols are essential in fungi, anti-fungal agents which uniquely inhibit the sterol transmethylases step might be clinically very useful. Cloning will provide increased enzyme from SMT expression vectors to screen and synthesize anti-fungal agents which specifically inhibit this step. Such anti-fungals may be less toxic to mammalian cells which lack this methylation reaction.